The present invention relates to certain compositions and methods useful for preventing and/or mitigating microbiologically influenced corrosion and staining occurring, for example, in a sealed environment.
Microbiologically influenced corrosion (MIC) of the internal surfaces of equipment such as pipes, tanks, and heat transfer components, in addition to non-MIC electrochemically influenced corrosion, results in extensive "remove and replace" maintenance projects. This is usually very costly and time consuming.
An alternative procedure used in some situations is to install an insert or lining, in situ, composed of a resin (e.g., epoxy or vinyl esters) system that forms a barrier between the host component, e.g. transmission pipe line, and the fluid being transported through the pipe line. Once in place, this lining provides a means of mitigating the effect of the corrosion. However, it may not prevent the corrosion mechanism from continuing to degrade the host component.
Wide-spread forms of MIC occur where anaerobic conditions exist. MIC is often identified as including pitting corrosion or "under-deposit" corrosion.
Microorganisms involved include sulfate reducing bacteria (SRB) and Clostridium types which produce H.sub.2 S or H.sub.2 that attack the metal. The microorganisms' source of inoculum is virtually unlimited. However, the corrosion caused by their growth occurs only when specific conditions exist such as an oxygen free environment. The microorganisms can grow to very large populations in localized sites and attack the host component, e.g., metal, causing a very aggressive corrosion condition.
Furthermore, inserting a lining or barrier on the inner surface of a pipe line may provide the ideal anaerobic environment for microorganisms to grow and subsequently influence corrosion of the host component. Cleaning and removing the debris and corrosion by-products, found on inner surfaces where the lining would interface, is a necessary step to insure proper application. However, this cleaning typically will not eliminate the microorganism inoculum source. Even under the best case conditions where cleaning was exceptionally complete, and the installation of the lining was flawless, the potential for MIC to be initiated or to resume activity is very high.
A four year study made under actual plant operating conditions examined various corrosion mechanisms involved with typical service water system materials of construction. Corrosion coupons were included as part of the test samples. Some of the coupons were placed in a position where the insert liner was placed as a barrier to provide the host component protection from corrosion. A few of the liner samples were intentionally flawed to simulate installation problems. Unlike actual plant installation of the liner, however, the base materials were coated with an epoxy adhesive to insure the liner material was attached to the coupon. It was known that the epoxy adhesive was not a specific moisture barrier; however, it should have provided some additional protection against corrosion of the base material. Some observations made during the study that related to MIC included:
1. Coupons in stagnant, intermittent, and continuous flow positions were susceptible to MIC.
2. The lack of tuberculation formation does not imply MIC has not occurred.
3. If there is damage to the coating or lining and the felt is not impregnated with resin, water will wick through and promote corrosion of the base metal.
4. Due to the specific water chemistry of the test site, no significant corrosion under the liner occurred, but other sites with different chemistries could experience severe corrosion.
Another type of liner that creates an anaerobic environment is the liner used in swimming pools. These types of liners are affected by staining caused, in part, by facultative anaerobic, anaerobic, or microaerophilic microorganisms.
The staining can be described as intense black-brown or gray isolated spots, or more diffuse gray discoloration of the vinyl surface and blotchy in appearance. The discolorations are, for example, located below the water level usually on the sloping sides and on the bottom of the pools. Staining has been observed at depth of 2-3 feet and to 9 feet. In most cases, the staining was localized where the back-side surface of the liner came into direct contact with the cement/sand or cement/vermiculite base used to form the pool in-ground and upon which the vinyl liner was placed. Rarely was staining observed on water-side surfaces where the back-side surfaces were in direct contact with galvanized metal vertical walls, or poured concrete/concrete block vertical walls. Stereoscopic microscope examination of the water-side stained surfaces of the liner indicated that the stain was not specifically the result of substances adhering to or adsorbed onto the vinyl surface which was exposed to the pool water. In fact, these observations indicated that the stain appeared to originate on the back-side surface and diffuse through the vinyl, appearing as a disfigurement on the water-side surface.
Staining occurred in pools routinely treated with oxidizing biocides such as hypochlorite salts, tri/dichloroisocyanurate, and bromine compounds to control growth of algae and other microorganisms. It also occurred in pools treated with nonoxidizing algicides such as quaternary ammonium salts and polyquat compounds often used in conjunction with oxidizing biocides. These investigations indicated that the chemical characteristics of the pool water (such as pH, hardness, and alkalinity) had no correlation to the occurrence of the staining. Staining was observed most frequently on liners that had been in place for 5-15 years. The stains usually appeared gradually over that time. However, it was also reported that liners used to replace the stained liners became stained in the same general location within a period as short as one year after replacement.
In addition to promoting an anaerobic environment, the inherent physical characteristics of liners and the procedures used to install them provide many ideal opportunities for the growth of anaerobic, facultative anaerobic, or microaerophilic microorganisms which can lead to staining or MIC.
The need to prevent MIC or staining or to mitigate an existing MIC or staining situation is clear. The key to preventing or mitigating MIC or staining is to prevent the growth of the microorganisms responsible for the MIC or staining. This can be done by including into the susceptible environment a biocide-biostat with efficacy in controlling or reducing the growth of facultative anaerobic, anaerobic, or microaerophilic microorganisms. The chemical characteristics of the biocide-biostat should preferably have the following properties: long persistency, minimum water solubility, passive to composition materials of the lining system and the host component (e.g., metal), non-hazardous, and environmentally acceptable.
Accordingly, a goal of the present invention is to provide compositions capable of preventing and/or mitigating microbiologically influenced corrosion, over prolonged periods of time. An additional goal of the present invention is to provide a method for biostatically reducing the growth of anaerobic, facultative anaerobic, or microaerophilic microorganisms. Another goal of the present invention is to provide a method for preventing and/or mitigating microbiologically influenced corrosion. A further goal of the present invention is to provide a method for preventing and/or mitigating the type of staining described above.
Additional advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The goals and advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the above noted goals and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention provides biocide-biostat compositions useful in 1) controlling and/or reducing the growth of an anaerobic, facultative anaerobic, or microaerophilic microorganisms and 2) preventing and/or mitigating microbiologically influenced corrosion or staining. The compositions contain a borate salt, preferably calcium metaborate, barium metaborate, calcium pyroborate, or mixtures thereof.
The present invention also provides a method for preventing and/or mitigating microbiologically influenced corrosion, for instance, in a sealed environment, which comprises the step of adding or applying a composition of the present invention to an area susceptible to MIC, e.g., inside a pipe to prevent and/or mitigate corrosion.
In addition, the present invention provides a method for controlling and/or reducing the growth of an anaerobic, facultative anaerobic, or microaerophilic microorganism which comprises the step of adding or applying a composition of the present invention to an area susceptible to the growth of the microorganism. Further, the present invention provides a method to prevent and/or mitigate staining caused by a microorganism comprising the step of adding or applying a composition of the present invention to an area susceptible to staining to prevent and/or mitigate such staining.
It is to be understood that both the foregoing general description and the following detailed description are exemplary only and are not restrictive of the present invention, as claimed.